Square cathode for ion getter pumps



Dec. 8 19.70 M. KLOPFER ET AL 3,546,510

SQUARE CATHODE FOR ION GE'ITER PUMPS Filed Nov. 30, 1967 2 Sheets-Sheet l j' s F|G.3a FlG.3b

INVENTORJ ANTON MARTIN KLOPFE'R HORST FLUMERT AGENT Dec. 8,1970 KLQPFER ET AL T 3,546,510

SQUARE CATHODE FOR ION GETTER PUMPS Fi led Nov. so, 1967 2 Sheets-Sheef 2 F I 6.5a

FlG.5b

INVENTOR ANTON MARTIN KLOPFER HORST FLUNKERT AGENT United States Patent ware Filed Nov. 30, 1967, Ser. No. 687,064 Int. Cl. F04h 37/02; Htllj 1/50, 7/18 US. Cl. 313-7 9 Claims ABSTRACT OF THE DISCLOSURE An ionization pump with a cold cathode in the form of a rod, tube, or the like of polygonal cross-section.

The invention relates to a gas discharge device comprising an electrically-conductive getter material cathode of rod or tubular form, an anode ring of electrically conductive material substantially surrounding at least part of the cathode and to provide a cold-cathode gas discharge path between anode and cathode, and means for producing a magnetic field in said path substantially parallel to the longitudinal direction of said cathode. Such devices may be used as an ionisation gauge or as a getter ion pump.

One known apparatus of this type has a cathode rod of circular cross-section and interconnecting two cathode plates situated at the open ends of the anode ring and substantially perpendicular to the longitudinal axis of the cathode.

More recently it has been proposed to construct pumps by arranging a number of cells each comprising such apparatus in a matrix in such a way that neighbouring cells have common anode walls. The efiective pumping speed is then higher for a single cell. It is known that ions striking the cathode rod at a glancing angle in each cell promotes sputtering of the cathode material.

This invention aims at obtaining stable pumping at lower pressures in pumps and at ignition at lower pressures in ionisation gauges.

The invention provides a gas discharge device operating in a .magnetic field comprising an electrically-conductive getter material cathode of rod or tubular form having a non-circular external cross-section over at least a portion of its length, an anode ring of electrically conductive material substantially surrounding at least part of the non-circular cross-section portion of the cathode to provide a cold-cathode gas discharge path between anode and cathode, and means for producing a magnetic field in said path substantially parallel to the longitudinal direction of said cathode. The rod may be solid or the tube may comprise abutting plates. The anode ring is not necessarily closed but is preferably at least substantially so because it is necessary that a closed equipotential surface be formed in operation around the noncircular cross-section portion of the cathode.

Preferably said non-circular external cross-section is polygonal, preferably square.

Preferably the internal cross-section of the anode ring has the same shape as said non-circular external crosssection and is preferably rotationally aligned with the external cross-section of the cathode rod about the longitudinal axis of the cathode.

Preferably the anode ring is made of a material to which sputtered cathode material can stably adhere in order that flaking oif of sputtered. cathode material from the anode should not occur in operation with the consequent danger of a short circuit occurring between anode and cathode. If the cathode is made of titanium the anode ring may be made of stainless steel.

Pumps comprising apparatus of a type according to the invention may show stable pumping of rare gases, especially argon, up to pressures as high as a few times 10- torr. On the other hand the discharge may remain stable at pressures lower than 10* torr and even at 10" torr the cold-cathode discharge may be started immediately in contradistinction to pumps and ionisation gauges having cathode rods of circular external crosssection or no cathode rods at all, which have been found to give considerable starting delay.

In order that the invention may be better understood, embodiments thereof will now be described, by way of example, with reference to the diagrammatic drawing accompanying the provisional specification and to the accompanying drawing in which:

FIGS. 1 and 2 represent a single cell of an apparatus according to the invention in elevation and plan respectively,

FIGS. 3a and 3b show cross-sections of an eroded cathode rod,

FIG. 4 shows in elevation a single row of a matrix of such cells, and

FIGS. 5a and 5b show an elevation and plan respectively of a practical embodiment of a matrix of such cells in more detail.

In FIGS. 1 and 2 a rod-shaped cathode 1 of square cross-section is arranged centrally within an anode ring 2 comprised by four flat walls rotationally aligned with the external cross-section of the cathode rod about the longitudinal axis of the cathode rod, i.e. so that the four inner faces of the anode ring face the four outer faces of the cathode rod. The solid cathode rod 1 interconnects and is supported at each of its ends by cathode plates 3 and 4 which are electrically connected thereto. The cathode 1, plates 3 and 4, and the anode ring 2 are arranged within an envelope (not shown) having a port for connection to a vacuum system which it is assumed is to be pumped. The anode and cathode are mutually electrically insulated and supported within the envelope. The cathode 1 is made of a reactive getter material such as titanium, zirconium, or tantalum and the anode ring 2 is made of a material to which the cathode material can readily adhere; if the cathode is made of titanium the anode may be made of stainless steel.

In operation a source of electric potential of several thousand volts is connected between the anode and cathode via a series resistance. Means are also provided for producing a magnetic field in the discharge path between anode and cathode parallel to the longitudinal axis of the cathode rod. This field may provide a magnetic flux density of, for example, several hundred gauss. The pressure in the vacuum system and the envelope is assumed to be sufficiently low initially so that an electric cold- 3 cathode discharge commences between anode and cathode. Cathode sputtering then ensues with consequent pumping action in known manner.

When the gas to be pumped contains argon a wellknown phenomen or with prior ionisation pumps is that known as argon instability. This is a slight pressure variation due inter alia to argon being first pumped to a cathode surface by a process which is believed to be that of covering argon atoms by reactive metal particles, sputtered into the cathode, while continuing disintegration of the cathode as ionisation proceeds releases these argon atoms with a consequent pressure rise. A sequence of such cycles follows with inability to pump argon stably. The difficulty arises because argon, being a noble gas, cannot combine chemically with the reactive getter material.

It has been found that the use of a cathode rod of square cross-section as described results in an ability to pump argon in a substantially stable manner at pressures up to torr, and at a speed which is 12% of the speed at which the same pump will pump nitrogen. It

has also been found that such a shape of cathode enables pumping to proceed to ultimate pressures below 2 1O torr.

With a cathode rod of square cross-section it has been found that sputtering of the cathode does not occur un1- formly over the cathode rod surface. Such a cathode eventually becomes eroded at areas denoted by 5 in FIG. 3 to a cross-section of the form shown on an enlarged scale in FIG. 3a, this type of cross-section being representative of a major portion of the cathode rod. The end plates 3 and 4 do not tend to disintegrate, in contradistinction to prior art pumps with end cathode plates such as 3 and 4 but without the cathode rod. Whereas prior art pumps have these and the cathode rods customarily of a reactive metal such as titanium, in embodiments of this invention the cathode plates 3 and 4 may as an alternative be of stainless steel. They may as an alternative be omitted altogether, the cathode rod then being supported by other means and having a suitable electrical connection to the electric potential source.

Although the cathode has been described as a rod, other forms are possible which provide an effective non-circular cross-section but which are not solid but hollow. For example, the cathode may consist of a tube which may be in one piece or be in composite form constructed from strips.

Although the external cross-section of the cathode rod has been shown as square it may, as an alternative, be in the form of any other polygon, regular or otherwise. In fact it is thought that the improved operation of apparatus according to the invention can be obtained with a cathode rod or tube which does not have any fiat faces at all, as long as it is not circular in external cross-section. It is thought that the improved operation of apparatus according to the invention results from the increased electric field occurring in operation adjacent those parts of the cathode rod or tube which have the smallest radii of curvature. Thus, preferably, the external crosssection of the cathode rod or tube exhibits one or more discontinuities.

A plurality of cells such as are shown in the FIGS. I and 2 may be arranged in the form of a matrix. One row of the cells in one such matrix is, shown in elevation in FIG. 4. In FIG. 4 similar items bear the same reference numerals as do their equivalents in FIGS. 1 and 2. An economy in construction of the anodes arises if, as shown, the adjoining anode walls of neighbouring cells are combined so as to give single partitions between adjacent cells. Such anode constructions are well-known for ionisation pumps. As shown in FIG. 4, the end cathode plates for each cell may be extended so as to join the corresponding plates of the adjacent cells, this resulting in a pair of parallel plates for the whole matrix.

Referring again to FIG. 3a which shows a cross-section of a sputtered cathode as in FIGS. 1 and 2 or individual cathodes shown in FIG. 4 it will be noticed that the sputtered parts 5 do not occur symmetrically on each face of the cathode but are displaced to one side thereof. The depth of sputtering is determined inter alia by the time the pump has been used and when sputtering has proceeded to an appreciable depth compared with the cathode thickness it is possible, by reversing the direction of the applied magnetic field, to cause sputtering to take place at fresh cathode areas to produce a new erosion pattern 6 on the other side of each cathode face. This new erosion pattern is shown dotted in FIG. 3b. The cathode rods may thus have their operative life extended by an approximate factor of 2.

The practical embodiments shown in FIG. 5 comprises a matrix of cells each having a cathode 1 and an anode ring 2. The cathode plates 3 and 4 extend over the whole of each side of the matrix and support the cathode rods 1. The cathode rods 1 and the plates 3 and 4 may be made, for example, of titanium while the anode rings 2 may be made of stainless steel. The cathode plates 3 and 4 are fixed together by means of channel members situated one at each end of the matrix. Each channel member supports a pair of electrical insulators 16 which may be made, for example, of ceramic material. The insulators 16 are in turn fixed to the anode matrix 2 so that the anode matrix is supported within, and insulated from, the pair of plates 4. Cup-shaped shields 17 are placed round each insulator so as to shield the insulator from sputtered material which may emerge in operation from the open ends of each anode ring.

The dimensions of one practical embodiment constructed as shown in FIG. 5 were: overall module thickness 34 mm., thickness of anodes mm., 36 anode rings each a square of mm. side (in a matrix 3 x 12), crosssection of cathode rods a square of 2.5 mm. side, length and width of module (including channel members) 328 mm. and 92 mm. respectively. In operation a magnetic field was applied perpendicular to the plane of the paper of FIG. 5b giving a flux density of 1700 gauss, and an electrical potential difference of 5 kilovolts was applied between the anode and cathode assemblies. With these values a maximum pumping speed for nitrogen (at about 2 10 torr) of 40 litres per second was obtained and pumping continued down to an ultimate pressure of below 2X 10- torr. The same embodiment was found to pump pure argon at pressures up to 5 l(ltorr completely stably at a maximum speed of 8 /2 litres per second.

What is claimed is:

1. A gas discharge device especially an ion pump or ionisation gauge comprising an electrically conductive getter material cathode of rod or tubular form having a square external cross-section over at least a portion of its length, an anode ring of electrically conductive material substantially surrounding at least part of the square cross-section portion of the cathode to provide a coldcathode gas discharge path between anode and cathode, and means for producing a magnetic field in said path substantially parallel to the longitudinal direction of said cathode.

2. A device as claimed in claim 1 wherein the internal cross-section of the anode ring has the same shape as said square external cross-section.

3. A device as claimed in claim 2 wherein said internal cross-section is rotationally aligned with said external cross-section about the longitudinal axis of the cathode.

4. A device as claimed in claim 1 including a cathode plate of electrically conductive material situated at each open end of the anode ring and substantially perpendicular to the longitudinal axis of the cathode, the cathode rod or tube inter-connecting said plates.

5. A matrix of cells each comprising a device as claimed in claim 4 wherein the cathode rods or tubes are substantially parallel to one another and the walls of adjoining anode rings are combined.

6. A matrix as claimed in claim 5 wherein the cathode Referen es Cited for the matrix. 3,233,823 2/1966 Asamaki 3137X 7. A device as claimed in claim 1 wherein the getter 3377499 4/1968 Lloyd material is titanium.

5 8. A device as claimed in claim 1 wherein the anode JAMES LAWRENCE Primary Examiner ring is made of stainless steel, C. R. CAMPBELL, Assistant Examiner 9. A device as claimed in claim 2 wherein said internal cross-section is at least 10 times the size of said external cross-section. 10 -153 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patmm No. 3,546,510 U Datmi December 8, 1970 n Anton M. Klopfer and Horst Flunkert It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, after line 6, insert as a new line --Claims priority, application Great Britain, December 10, 1966, 55527/66-;

Signed and sealed this 6th day of April 1! (SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents 

